docs: def _assert_gcs_files(files):
    """Check files starts wtih gs://.
  
    Args:
      files: string to file path, or list of file paths.
    """
  
    if sys.version_info.major > 2:
      string_type = (str, bytes)  # for python 3 compatibility
    else:
      string_type = basestring  # noqa
  
    if isinstance(files, string_type):
      files = [files]
  
    for f in files:
      if f is not None and not f.startswith('gs://'):
        raise ValueError('File %s is not a gcs path' % f)